Subgingival infection with Actinobacillus actinomvcetemcomitans (A.a.) is closely associated with juvenile periodontitis. A.a. strains exhibit variable leukotoxin production with some strains making 10 - 20 times higher levels of leukotoxin than others. These highly toxic strains have two promoters directing leukotoxin expression, as well as a small open reading frame encoding a 78 amino acid peptide. Strains expressing lower levels of leukotoxin have one promoter and an additional 530 basepair region. In this study, with Dr. Joseph Zambon, both a dot blot and a polymerase chain reaction assay were used to distinguish between highly toxic and minimally toxic A.a. strains. More than 200 strains were examined from over 130 human subjects diagnosed with either localized juvenile periodontitis (LJP), adult periodontitis (AP), or who were periodontally normal (N) as well as strains from non-human primates. Only LJP subjects, approximately half, were infected with highly toxic A.a. strains. The AP, N, and non-human primates did not harbor highly toxic A.a. Looking at multiple isolates, subjects had either highly toxic or minimally toxic strains but not both. The mean age of subjects harboring highly toxic A. L was significantly lower than for subjects harboring minimally toxic strains. Together, this data suggests that LJP is associated with infection by highly toxic strains of A.a.